Lennon followers laid wreaths outside the New York apartment block where he was shot on December 8, 1980.
Fans began a candlelight vigil in the nearby Strawberry Fields section of Central Park ahead of 10:50 pm, the time when Lennon was shot by Mark Chapman on the steps of the Dakota building as he was returning to his apartment with wife Yoko Ono.
Chapman had asked for Lennon's autograph hours before shooting him. "Mr. Lennon!" he shouted before aiming a handgun and firing five bullets at the musician.
Liverpool remembers
In the English city of Liverpool, where Lennon was born 65 years ago, more than 1,000 messages to the ex-Beatle, including emails from across the globe, were to be released on white balloons from the city's Albert Dock.
"I just wrote 'Merry Christmas John' on my balloon," said nine-year-old English fan James Andrews, one of the youngest to take part in the commemoration.
A short service was held at Lennon's statue in the city, near the Cavern Club where The Beatles played some of their earliest gigs. A memorial service was also to take place at Liverpool Parish Church.
On the eve of the anniversary, Lennon's widow, Yoko Ono, was one of four people honoured at the annual dinner of the New York chapter of the National Academy of Recording Arts and Sciences.
"This week has been hard," the 72-year-old Ono was quoted as saying. "Depressing. Now I'm smiling. I'm here. I see all your faces. John would have been so happy that you acknowledged his achievements."
Musical tributes
Japan marked the anniversary with special radio broadcasts and a free concert at a museum in the Tokyo suburb of Saitama that was built with Ono's blessing.
Concerts were also held in Britain and the United States.
The US Sirius Satellite Radio network hosted a four-hour "Lennon Live" tribute featuring performances from rock and pop stars Dave Matthews, Paul Weller, Dr. John, Daryl Hall, Stereo MC's and Lulu.
A quarter century after Lennon's death, his family, friends and fans are still struggling for control of his legacy.
Defining Lennon has become an industry in itself with exhibitions, memoirs, album re-issues, documentaries, a Broadway musical and a looming film.
Yoko Ono remains the gatekeeper of the Lennon legacy and chief protector of his posthumous image.
She also marked the anniversary by editing a compilation of tributes from celebrities such as Elton John and Mick Jagger.
But in the run-up to the anniversary, other voices offered a differing view of Lennon.
In her memoir "John," published in October, Lennon's first wife, Cynthia Lennon, wrote of the "pain, torment and humiliation" she suffered in her marriage.
And in a statement for the 25th anniversary, Lennon's son, Julian, admitted to "very mixed feelings" about his father.
"He was the father I loved who let me down in so many ways," he said.
Parole denied
Meanwhile, Chapman is still serving a 20 years to life prison sentence for the murder.
He’s locked up in the Attica maximum security facility in New York state and has had parole appeals turned down three times.
Chapman was obsessed by the former Beatle and craved his popularity.
In an interview recorded more than 10 years ago, Chapman described how the search for his own personality compelled him to shoot Lennon.
"I was under total compulsion," Chapman said in remarks recorded by a reporter in 1991 and 1992 that Britain's Channel 4 television will broadcast publicly for the first time this week.
"It was like a train, a runaway train, there was no stopping it. No matter -- nothing could have stopped me," he said.
Born in Texas, Chapman had a difficult childhood when he said he was picked upon by other boys at school. While waiting for Lennon outside the apartment block, Chapman had read "Catcher In The Rye" the story of a mixed up teenager.
He told a parole board last year he had planned the murder three months in advance and had even travelled to New York once before returning home to Hawaii thinking he was no longer angry at Lennon.
But the anger overtook him again when he saw photos of the ex-Beatle. The former security guard and drug addict then retraced his steps back to New York and waited for his moment.
"There was a successful man who kind of had the world on a chain, so to speak, and there I was, not even a link of that chain, just a person who had no personality. And something in me just broke," he said.
"And I remember thinking perhaps my identity would be found in the killing of John Lennon."
Now 50, Chapman was denied parole for a third time last year after the board cited the "extreme malicious intent" of his crime and said releasing him would undermine respect for the law.